This invention relates to a printed circuit or hybrid circuit board with a formed solder deposit on a surface mount device (SMD) pad thereon, and for the formed solder deposit itself.
Various methods are known for applying solder selectively to printed circuit boards (PCBs) in such a way that following the assembly of the boards with SMD components, the boards can be electrically and mechanically connected to the components by reflow soldering methods.
In particular, the prior art uses a method in which solder is deposited on selected regions of the PCB by screen printing or stencil printing with soldering pastes. These selected regions of the PCB, the pads, have usually been hot air solder leveled leaving only a thin layer of solder inadequate to form a joint. Alternatively, the copper pads may have been treated with an anti-tarnish material which prevents oxidation of the copper promoting adhesion of solder thereto. Eventually, this anti-tarnish material burns off during subsequent heating.
This method is in common use today but there are many problems associated therewith. These problems are not only those associated with screen printing such as investment cost, wear, plugging, and resolution, but also the solder forms "bumps", that is, the solder has a more or less convex cross-sectional shape making placement difficult and contributing to rework. See FIG. 1 wherein (1) is a solder mask, (2) is solder, (3) laminate, and (4) a pad. Thin solder deposits are often produced with low or no mechanical and thermal load-bearing capacity for the solder connections.
Another conventional method is immersion application of solder wherein a prepared PCB, that is one having solder resist thereon, is dipped into and removed from a solder bath. After removal and cool down, the metallized regions of the PCB are provided with solder deposits, which, however, also suffer from the formation of "bumps". The height of the solder deposit is dependent on the dimensions in the plane of the PCB of the regions to which solder is to be applied, when such regions have different dimensions, solder deposits of variable height necessarily result.
U.S. Pat. No. 5,051,339 issued Sep. 24, 1991 to Friedrich et al., the so-called "OPTIPAD" process, is an attempt to overcome some of these disadvantages. This is an immersion process, thus there are no foreign ingredients, only solder in the solder deposits. This patentee points out that hot air or hot oil leveling is not considered to be close prior art, whereas immersion soldering without ensuing air leveling is.
The process involves a PCB having thereon a solder mask with pads exposed, laminating thereto a temporary photoimageable layer of perhaps 5 mil in thickness, exposing and developing so that everything is masked but the pads to be soldered, immersing the thus prepared PCB in molten solder, and then contacting the board with a closure element to maintain the solder in place until it solidifies thereby flattening it.
The coating of the PCB with a temporary top solder mask is indispensible to the invention. The closest prior art did not contain the step of covering, with at least one closure element at a defined contact pressure, the voids located above the regions to which solder is to be applied when filled in the soldering bath with liquid solder. Stripping the temporary layer leaves behind a flattened 5 mil high pillar of solder. Aside from the need for expensive equipment which is not commercially available, this process has two major problems: (1) when the costly temporary coating sees molten solder it cures extensively and is difficult to remove even when stripping with caustic soda which has the other effects of not only dulling and oxidizing the solder, but of attacking the permanent mask; (2) when the 5 mil solder mask is stripped it leaves pillars of solder which in fine pitch applications when mated with their components collapse and generate extensive shorts. Any attempt to reduce the thickness of the mask in order to reduce the height of these pillars results in a greater degree of curing and an even more difficult stripping operation.
Others are modifying this process by screening molten solder into the pad wells. Screening molten solder not only has the same problem but there is severe dulling of the solder probably shortening storage life due to intermetallic phase formation and oxidation.
In addition to the '339 patent mentioned above, there are two other relevant publications; a paper by W. J. Maiwald of Siemens entitled, "Reliable Reflow Soldering Techniques using Preformed Solid Solder Deposits, Part 2--The Assembly Process" and the associated paper by M. Weinhold of DuPont entitled, ". . . Part 1--The Printed Circuit Fabrication Process".
This "SIPAD" (Siemens) process applies solder paste onto boards with permanent solder masks, melting the paste and flattening the round, humped solder deposits by a thermal/mechanical process. See FIG. 2 wherein (5) is a pressure plate.
"SIPAD" also requires highly specialized equipment although it can be run in a conventional multilayer press normally used for PCBs. However, this type of press requires about 2000 psi before activation, and though the platens see the top of the solder first, they then contact the PCB itself and thermally shock the laminate, oftentimes scorching and mechanically damaging the solder mask as well. Furthermore, the solder when compressed, squeezes sideways as a very thin film or foil. While this problem can be solved by various techniques to remove the film, it results in extensive and expensive rework and fine droplets of the thus squeezed out solder end up as solder balls.
Since solder "wicking" can be either sideways or upwards, in order for "SIPAD" to achieve maximum densities on fine pitch, the design of the pads must be changed by elongating them. An integrated company can alter their designs but most manufacturers cannot. Wicking upwards is, of course, desirable; sideways is not.
The major problems which both of these processes have attempted to address are the following:
1--opens and shorts due to squeezing out of solder and resultant solder bridging; PA1 2--low packing density and inability to solder with high pin counts without extensive design modifications; PA1 3--the printing of solder paste; PA1 4--achieving the required shape of the deposit; PA1 5--presence of a "bump", the meniscus, makes positioning of fine pitch components difficult resulting in unacceptable tolerances; PA1 6--solder balls; PA1 7--shelf-life of solder joint, short storage times; PA1 8--poor solderability due to too thin deposits; (Hot Air Solder Leveling generally leaves thin deposits, with consequent growth of an intermetallic phase preventing wetting of the SMD solder pads during reflow or wave soldering.) PA1 9--poorly defined soldering gap; PA1 10--inability to quantify and standardize solder deposit and solder gap; PA1 11--yield after soldering; (First-pass yields in standard applications are running at 60-70%, while for many fine pitch cases they are only 10%; rework is extensive.) PA1 12--overall cost; and PA1 13--quality of board and solder joint. PA1 1--removal of the solder paste printing process from the assembler's operation; (The PCB fabricator can supply to the assembler circuits with a solid, flat solder deposit. New processes can be used to replace solder paste printing and which obviate the need for cleaning the PCB assemblies without the risk of solder balls or other contamination.) PA1 2--guaranteed reflow solderable PCBs; PA1 3--unlimited good solderability of the PCB; PA1 4--small solder structures; PA1 5--a defined solder gap; solder in a defined three-dimensional well, i.e., an excellent profile; PA1 6--assembler would then use a 100% tested presoldered board since faults arising from solder application can be separately controlled, eliminated or reworked at the PCB fabricator without the obstruction of components; PA1 7--the problem of solder paste deposits being deformed when the component terminals touch down is non-existent with this method; (Components can be placed on a flat surface which would permit the use of fine-pitch flat packs and TAB assembly with automated equipment.) PA1 8--finer PCB structures can be implemented; PA1 9--practically no shorts; PA1 10--no solder balls; PA1 11--too thin solder layers are not encountered; PA1 12--fluxing agents can be optimized; PA1 13--unlimited shelf-life between placement and soldering; longer guaranteed storage time; reduction in growth of the intermetallic phase resulting in higher peel strength; PA1 14--the possibility of quantifiable and standardized solder deposits and solder joints; PA1 15--better yield after soldering with considerable reduction of rework; higher first pass yields; PA1 16--better overall quality of boards and solder joints; improved product consistency; and PA1 17--lower cost due to faster throughput in assembly with shorter SMD assembly lines.
The solutions and benefits both of these approaches have attempted to achieve are as follows:
Neither of these two processes have been especially successful.
The "SIPAD" process uses solder in the well to solder, not the pillar of "OPTIPAD"; the process of this invention uses both.
In summary, both of these new processes leave much to be desired and are not industrially practical in their present state of development. As a consequence, the study which resulted in this subject invention was undertaken.